Extreme Ultra-Violet Lithography (EUVL) is a leading emerging technology for 13 nm mode and beyond for the production of Micro Processing Unit and Dynamic Random Access Memory (MPU/DRAM) integrated chips. Presently, EUVL scanners which produce these Integrated Chips (ICs) are used on a small scale for purposes of evaluating and demonstrating this new technology. The optics systems, which include reflective optical elements, are an important part of these scanners. As EUVL development proceeds towards high volume manufacture, the specifications are expected to continue to become more stringent for the optics system parts.
In EUVL scanners, the optical elements are exposed to an intense extreme ultraviolet (EUV) radiation having a wavelength of 13.5 nm. Some portion of this EUV radiation is absorbed by the reflective coatings on the optical elements of the systems, which results in the heating of the top surface of the optical element. This causes the surface of the optical element to be hotter than the bulk of the optical element and results in a temperature gradient through the optical element. In addition, in order to image a pattern on semiconductor wafers, the surface of the optical element is not uniformly exposed to the EUV radiation, which leads to a temperature gradient on the optical surface. This results in a complex three dimensional temperature gradient through the thickness of the optical element, as well as along the optical element surface receiving the radiation. These temperature gradients lead to a distortion of the optical element, which in turn leads to smearing of the image being formed on the wafers. It is expected that the difficulties of heat dissipation will be exacerbated by the increased optical element sizes and the increased power levels that are anticipated to meet the demands of future EUVL developments.